In optical fiber manufacturing processes, fiber is drawn from a large-diameter glass structure known as a preform. Processes for making a preform include outside vapor deposition (OVD) and vapor axial deposition (VAD). In such processes, a porous glass structure known as a soot body or soot core is formed by depositing particles onto a glass seed rod (also known as a bait rod). The soot body is then condensed into a more solid structure known as a core body. The core body is then further processed into a preform.
In OVD, gases such as silicon tetrachloride, oxygen, and hydrogen, are reacted in a torch flame, and the resulting particles, known as soot, are deposited on the seed rod. The reaction occurs along the gas stream until a portion of the soot is deposited. Uniformity of the deposited soot is desirable and depends on many factors. The torch may include multiple orifices. For example, the torch may include a central tube or orifice as well as one or more concentric tubes or orifices surrounding the central orifice. The various orifices may be configured to emit different gases, some of which are reactants or reaction-supporting gases, and others of which may be shield or inert gases that help contain or otherwise control the reaction. The torch may be traversed by a mechanical carriage back and forth along a rotated seed rod until a sufficient amount of soot is deposited. It is very important that the soot be uniformly deposited. If the traverse is too fast, soot may build in a helical bubble or non-uniformity on the surface of the structure. If the traverse is too slow, soot may build in an undesirably thick and non-uniform manner on the surface.
Linear burners or torches having a length corresponding to the length of the seed rod have been suggested for use in OVD. A linear torch may have multiple orifices evenly distributed along its length. Alternatively, a linear torch may have a grating comprising slit-shaped openings extending substantially along its length that similarly serves as an orifice. A linear torch directed generally at the exposed semi-cylindrical surface of a seed rod may undesirably deposit soot more thickly at points closer to the torch and more thinly at points farther from the torch.
In VAD, the torch is directed at the base of the seed rod, and the seed rod is gradually drawn away from the torch as the soot builds up. Although VAD may have certain advantages over OVD, VAD may be more difficult to control with sufficient precision than OVD, due to the tapering diameter of the soot growth surface at the base. Known linear torches may be poorly suited for VAD because they may not deposit soot uniformly.